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High School/JEE/NEET/IPhO Physics | 17-19 Yrs

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  1.  
    2. Sir why is the work done by friction equal to zero in case of pure rolling!
    2 years ago by Ayush Mangal

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  2.  
    3. Respected Sir, I have a doubt that you have considered static friction in pure rolling as you have mentioned in your previous slide that we are suppose to account friction whenever there is an Fext.My doubt is that which external force is accounted here is force F on whole system?
    2 years ago by Umang Kumar

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  3.  
    4. Sir what if there is a tilted disc in place of sphere then how to find KE of disc??? I think we can't apply 1/2(I)w^2 formula there because its not principle axis of rotation.
    2 years ago by omprakash nain

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  4.  
    5. sir while calculating velocity friction is not considered but while calculating acceleration we have considered friction can you please explain this
    2 years ago by

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  5.  
    6. Sir, you said that Perpendicular Axes Theorem is only applicable for laminar bodies, but a cube isn't laminar so how did you use it on a cube at the end of the video?
    2 years ago by

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  6.  
    7. Sir I had one doubt..... we are assuming that the friction on the bottom particle in the case of pure rolling is zero coz of the 0 relative motion due to which Vc and Rwc balance each other out. So how does pure rolling eventually come to an end in real life situations..... its not possible for pure rolling also to continue forever
    2 years ago by

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  7.  
    8. Thank u sir
    2 years ago by

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  8.  
    9. Sir is complete content of RBD is given in this playlist or some topics are left
    3 years ago by Jayraj

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  9.  
    10. No Question is difficult if it is analyzed carefully .. After learning sequenced theories, Such questions are really boosting my confidence sir.. Thanks a million for such quality content respected sir!
    3 years ago by

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  10.  
    11. Sir will we do instantaneous axis of rotation for rigid body dynamics
    3 years ago by Priyanka Ghosh

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  11.  
    12. sir why you did not considered initial K.E . OF A SPHERE AS IT IS ROLLING initially
    3 years ago by charu sahu

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  12.  
    13. why didn't you take translation k.E. into account?
    3 years ago by shreyash gaikwad

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  13.  
    14. sir voice is not coming in any of your videos
    3 years ago by saitama double

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  14.  
    15. why is velocity wrt wedge is taken while writing rotational k.E. instead of ground frame.
    3 years ago by shreyash gaikwad

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  15.  
    16. Sir is this relation directly valid even if mass distribution of body is not uniform ?
    3 years ago by Kushaldeep Singh

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  16.  
    17. sir in this q we know after attaining maximum velocity the ball will continue the circular path ,so here the centripetal force will be provided by the spring force only(provided smooth surface)..so if we write an equation of centripetal force we get the value of V²(max)..but with this relation the Ans that I arrived doesn't match with urs..sir pls rectify my mistake ...pardon
    3 years ago by N K D

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  17.  
    18. sir pls can you tell at 5:36 why will the axis of rotation apply some force on disc pls tell sir
    4 years ago by Yash Veer Nagar

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  18.  
    19. Sir when writing the kinetic of sphere at vertical AOR through point O, at time of writing of moment of inertia, shouldn't we write it is 2/5 mr^2 + m(R+r) ^2 ?
    4 years ago by Vidhi Patidar

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  19.  
    20. sir where is the video related to rotational collisions. topics like hinged and free collisions are included in this course?
    4 years ago by Eklavya Goyal

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  20.  
    21. How angular momentum is conserved, if torque due to mg is acting in frame of that point?
    4 years ago by Shivansh Garg

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  21.  
    22. sir the torque acting is zero as the com is not moving (as it is at rest) and all forces act through centre of mass so the hinge force balances the friction force overall. CAN YOU PLEASE TELL ME IF I AM RIGHT OR WRONG.and if wrong please give me the explantion for troque being zero
    4 years ago by sanmit kedar

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  22.  
    23. sir the torque acting is zero as the com is not moving (as it is at rest) and all forces act through centre of mass so the hinge force balances the friction force overall. CAN YOU PLEASE TELL ME IF I AM RIGHT OR WRONG.and if wrong please give me the explantion for troque being zero
    4 years ago by sanmit kedar
    Ans 1 ->
    module 24 example. I am sorry, posted it here by mistake
    4 years ago by sanmit kedar

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  23.  
    24. sir what would be the important topics for 2021
    4 years ago by

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  24.  
    25. Sir can you advise me what to do after seeing your video lectures. That is, how to strengthen my concepts further as i do not feel confident with some concepts and problems..
    4 years ago by Sanjeev VA

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  25.  
    26. Sir where you discussed about torque???
    4 years ago by Raja Singh
    Ans 1 ->
    Yes i am also having same question.
    3 years ago by Swapnil Acharya

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  26.  
    27. Sir how have you considered friction as internal force and how angular momentum is conserved?
    4 years ago by Pranav

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  27.  
    28. Can we conservation of energy equation also?
    5 years ago by undefined

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  28.  
    29. sir as coeff.of restitution is e ...so after collision the ball may be in motion ...
    5 years ago by justin maurya

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  29.  
    30. Sir please explain how integration of dm.r.cos(theta) is zero at 06:30, As mass moment is equal to dm.r
    5 years ago by Abc

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  30.  
    31. Sir, In case 4 i.e. Angular momentum of a rotating body about an AOR passing through COM and moving translationally. L(C) was around AOR, so how can this be taken same around point P. As if we take Velocity to be zero in final expression it will suggest L (P)=IW. Please explain.
    5 years ago by Tarik

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  31.  
    32. sir,where is torque force?
    5 years ago by Arun Prasath

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  32.  
    33. Thank you sir, for providing such a good set of questions.
    5 years ago by Adarsh Mall

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  33.  
    34. Sir, in a collision, can we always conserve linear and angular momentum?
    5 years ago by Adarsh Mall

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  34.  
    35. Sir, in a collision, can we always conserve linear and angular momentum?
    5 years ago by Adarsh Mall

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  35.  
    36. sir there are only 5 months for jee adv and iam not so good with the preparations untill now. will i get enough time to practice sufficient amount of problems for jee after watching all these videos. iam aiming good rank in jee adv for iisc. sir please help me
    5 years ago by Car Nival

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  36.  
    37. Sir in the last equation of forces "2mg N = m(v1)^2/R" shouldn't we take 2v1 instead of v1 as at the uppermost point the velocity of mass will be (velocity of com) (wr) and as it is pure rolling both are equal hence the velocity at topmost point should be 2v1.
    5 years ago by undefined

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  37.  
    38. Sir,why is torque calculated about an axis while angular momentum is measured about a point?Please explain.
    5 years ago by Manasi Singh

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  38.  
    39. Sir why have you included the child's MOI while calculating the recoil angular momentum of the platform?
    5 years ago by Aman

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  39.  
    40. Sir why have you included the child's MOI while calculating the recoil angular momentum of the platform?
    5 years ago by Aman

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  40.  
    41. Sir why have you included the child's MOI while calculating the recoil angular momentum of the platform?
    5 years ago by Aman

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  41.  
    42. Why can't we use the formula discussed in the fourth case for axis not passing through the centre of mass?
    5 years ago by PIYUSH MISHRA

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  42.  
    43. Sir in this case why don't we consider moment of inertia of man about centre
    5 years ago by Aditya Panpaliya

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  43.  
    44. why can't we apply mass distribution property here. can't we use expression calculated for dic along diametric axis??
    5 years ago by tukun saini

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  44.  
    45. but for rolling motion kinetic friction is zero.Is the friction considered in the problem the static friction???
    5 years ago by Shark Roy

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  45.  
    46. sir why you have taken elemental mass as M/L.dx and not (MASS/VOLUME).VOLUME OF ELEMENTAL MASS??
    5 years ago by Divit Goyal

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  46.  
    47. The moment of inertia of a square plate, a triangular lamina , etc are not mentioned in ncert. Are they still asked in jee? Also , i am not able to choose elements to integrate , that is , i am facing a huge problem while solving problems using calculus. Can you please help?
    5 years ago by Mitesh Katariya
    Ans 1 ->
    They don't ask directly, although u should know it!!
    5 years ago by rishabh saraswat

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  47.  
    48. How come both the tensions are equal?
    5 years ago by Vamsi Krishna

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  48.  
    49. Sir you said no external force is present ao can we use conservation of linear momentum
    6 years ago by Kunal Verma

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  49.  
    50. why we didn't integrated it on the surface from 0 tom (r^2 H^2)^0.5 and rather integrated from 0 to H.
    6 years ago by Mayank Goyal

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  50.  
    51. How can we consider friction on the pulley, as it is mentioned as ideal??
    6 years ago by Muskan jain

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  51.  
    52. sir I never understand how to find M .I. of any body
    6 years ago by Satyavan Pal

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  52.  
    53. Sir for the rotational part too the equation should be fR=((MR^2)(a2-a1))/2r because a1 is acceleration of cylinder w.r.t the plank not w.r.t the ground.
    6 years ago by Bala Rajput

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  53.  
    54. this is beautiful
    6 years ago by Harsha Indukuri

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  54.  
    55. sir, the force due to friction acts on every point on the elemental ring and not just on one point.....so shouldn't we take a complete integral of friction acting on every point on the ring rather than that on one point only.?
    6 years ago by Rishabh Chaurasia

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  55.  
    56. Sir but if a point mass revolves around an aor so the radius of circle described will be constant throughout then what will be the other factors affecting I other than mass
    6 years ago by Naganathan Iyer

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  56.  
    57. But sir if the platform was moving in the other direction , won't the relative velocity of the boy change?
    6 years ago by K.S.Sreevatsan

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  57.  
    58. sir in acceleration down the inclined plane of the body since point of contact should be at rest why is friction acting on it . please reply soon.
    6 years ago by
    Ans 1 ->
    It is a example of pure rollong so here only static friction is taken in consideration and because body is accelerating down the plane, friction is taken into consideration.
    6 years ago by Digesh Gaundar

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  58.  
    59. For pure rolling we can consider any direction of friction, result will remain same as it is static friction...
    6 years ago by

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  59.  
    60. Sir why did you divide with volume of solid sphere instead of dividing with area of solid sphere. I got this doubt because for hollow sphere you divided with area and for this you divided with volume. Hope you could reply me soon..
    6 years ago by Neelesh
    Ans 1 ->
    Bhai Neelesh kyunki hollow sphere pe mass surface area pe divided Retha hai but solid sphere pe mass volume pe divided Retha hai
    6 years ago by Sarthak Singh Chauhan

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  60.  
    61. Superb superb
    6 years ago by Neelesh

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  61.  
    62. It is nice
    6 years ago by Neelesh

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  62.  
    63. Best teacher
    6 years ago by Ashu

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  63.  
    64. The centre of mass of a disk ,in a vacuum surrounding,is given some velocity on a rough surface.Will the ball ever stop?
    6 years ago by Satwik Murarka

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  64.  
    65. Good
    6 years ago by Venkataramana

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  65.  
    66. I need all formula like recap of chapters...plzzzz...
    6 years ago by aswin gawtham

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  66.  
    67. SIR Is it necessary for the rollers to be in pure rolloing??
    6 years ago by TAMAJIT BANERJEE

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  67.  
    68. SIR Is it necessary for the rollers to be in pure rolloing??
    6 years ago by TAMAJIT BANERJEE

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  68.  
    69. Sir, Qusets will work better if you provide solutions at the end.Please provide skip option for the questions.Without it,we have to choose any option blindly and proceed for the next question.Kindly provide the solutions
    6 years ago by Pushpa Latha

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  69.  
    70. From where can I download brainstormer assignments and it's solutions
    6 years ago by

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  70.  
    71. sir i am confuse why you have multiply by 2/3 what is the relation b/w hollow sphere and solid sphere so that you have written 2/3integrate dmR^2
    6 years ago by
    Ans 1 ->
    because the element is considered to be a hollow sphere
    6 years ago by Biswarup Mukherjee

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  71.  
    72. sir why we are not considering the work done by friction and work done by rotational kinetic energy and sir it is mentioned in book word done by static friction is zero only in fixed surface but here the inclined plane is moving with some velocity..........please reply sir
    6 years ago by

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  72.  
    73. sir why we are not considering the work done by friction and work done by rotational kinetic energy and sir it is mentioned in book word done by static friction is zero only in fixed surface but here the inclined plane is moving with some velocity..........please reply sir
    6 years ago by

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  73.  
    74. I am feeling confused why the speed was (u-wR). As per question, boy has u relative to platform ie. when boy jumped his speed u was greater than that of platform. For eg. let boy had x speed wrt ground. so x-wR=u ie. x=u wR. I don't know in what case we should use relative velocity and in which case we should take absolute velocity. Can anyone guide me?
    7 years ago by Rupesh Koirala

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  74.  
    75. sir how can you say that the sphere does not move
    7 years ago by santhi sree kayala

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  75.  
    76. sir the question is A thin uniform hemispherical bowl of mass m and radius R is lying on a smooth horizontal surface,A horizontal force F is applied perpendicular to the rim of the bowl,what is the instantaneous angular acceleration of the bowl?
    7 years ago by Yash Awasthi`

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  76.  
    77. sir can we say that since axis of rotation and mg (force exerted by the block) are parallel therefore net torque by the block becomes zero because sin (theta) becomes zero so angular momentum can be conserved about the axis of rotation
    7 years ago by Aryan Gupta

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  77.  
    78. sir can we say that since axis and mg (force exerted by the block) are parallel therefore net torque by the block becomes zero because sin (theta) becomes zero
    7 years ago by Aryan Gupta
    Ans 1 ->
    therefore angular momentum can be conserved about that axis
    7 years ago by Aryan Gupta

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  78.  
    79. How do you know when Torque external is 0 or not?
    7 years ago by Aditya Kelkar

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  79.  
    80. How do you know which direction the torque is in? clockwise or anticlockwise?
    7 years ago by Aditya Kelkar

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  80.  
    81. how net torque 2TR comes??????
    7 years ago by anwesha singh
    Ans 1 ->
    Dear friend. Before finding torque we should know about which point/axes it is calculated about. here torque is taken about axes of cylinder i.e. make a diagram with cylinders circular face as front view in your paper. then u will und. 2 forces (T&T) are acting at a distance R from centre in upward direction. thus net Torque is 2TR. hope its clear
    7 years ago by ashish N

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  81.  
    82. sir in pure rolling if point of contact is at rest how does motion takes place?why work done by friction in pure rolling is zero? please answer
    7 years ago by HARSHIT SHARMA
    Ans 1 ->
    Firiction in an internal force thats why work done by it is zero
    6 years ago by Digesh Gaundar

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  82.  
    83. sir how to use the code given in your pg book
    7 years ago by Sparsh agrawal

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  83.  
    84. Sorry sir i am wrong u did it correct.
    7 years ago by Pragyan vashishth

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  84.  
    85. Sir like u said r×v but u did v×r gave the wrong direction of angular momentum so look to it
    7 years ago by Pragyan vashishth

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  85.  
    86. Sir.Why wasn't linear (translational) kinetic energy considered
    7 years ago by VISHAL REDDY MANDADI

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  86.  
    87. instead of 2tr should it be 0?? because one T is producing clockwise other anticlockwise so tr-tr=0?/
    7 years ago by Aditi Gupta
    Ans 1 ->
    Dear friend. Before finding torque we should know about which point/axes it is calculated about. here torque is taken about axes of cylinder i.e. make a diagram with cylinders circular face as front view in your paper. then u will und. 2 forces (T&T) are acting at a distance R from centre in upward direction. thus net Torque is 2TR. hope its clear
    7 years ago by ashish N

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  87.  
    88. Sir why did you took friction smaller than limiting friction??
    7 years ago by Advait Kulkarni

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  88.  
    89. sir in which topic coriolis acceleration will be discussed
    7 years ago by HARSHIT SHARMA

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  89.  
    90. respected sir...this was an undoubtfully helpful concept but i am facing problem to solve the above problem by conserving torque in the mid point of the plank.Please help me!
    7 years ago by Prateek Anand

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  90.  
    91. sir here we didnt took torque of mg beacause it was not involve in rotional motion of the cylinder.... that is the only reason or some else reason is there
    7 years ago by Raj Sancheti
    Ans 1 ->
    Yes as mg is not rotating the cylinder, we have not considered it...
    7 years ago by Physics Galaxy
    Ans 2 ->
    if distance b/w line of force (mg here) and axes of torque(cylinders axis here) is 0 then torque of the force about the axis is 0.
    7 years ago by ashish N

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  91.  
    92. Please let me know where can we put additional questions regarding your books which we do not get
    7 years ago by ramesh nagda
    Ans 1 ->
    goto INTERACT tab of our website www.physicsgalaxy.com and in the concerned topic, you can ask your queries...
    7 years ago by Physics Galaxy

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  92.  
    93. I am. It getting answer of your problem of your book mechanics fig 5.70 kindly help me
    7 years ago by ramesh nagda
    Ans 1 ->
    pl post your personal queries in the INTERACT tab of this website under specific topics... posting irrelevant queries below a video may distract other viewers... pl take care from next time...
    7 years ago by Physics Galaxy

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  93.  
    94. Why did we added I(rod)+I(sphere)=I(sphere w.r.t axis)?
    7 years ago by Akshunn Trivedi
    Ans 1 ->
    because in this case rod and sphere are attached as a composite body...
    7 years ago by Physics Galaxy

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  94.  
    95. Sir if we try to find the Moment of Inertia of the ring about the diametrical axis by integration I am getting the same answer. Actually I cannot find any difference as still the elemental mass would be making a circle of radius r just with a different orientation. Pls Help!!!
    7 years ago by Akshunn Trivedi
    Ans 1 ->
    If you are getting the same result then whats the problem and what help you need... in case of diametrical axis elemental mass dm is not always at the distance r from the axis... while integrating you need to substitute the radius of dm in terms of T before integrating...
    7 years ago by Physics Galaxy

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  95.  
    96. Won't there be frictional force acting on the cylinder? (It's not given that it is idea)
    7 years ago by Perin Jhaveri
    Ans 1 ->
    We do not consider unless specified in the question... here we assume that axle is frictionless...
    7 years ago by Physics Galaxy

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  96.  
    97. Sir pls can u solve 1.265 and 1.273
    7 years ago by Naman Biyani

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  97.  
    98. How is N2=(mu)N1?
    7 years ago by Aru Soni

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  98.  
    99. Sir there are no lectures on Instantaneous axis of rotation? Can you please upload it...!
    7 years ago by Vivek Sindhwani
    Ans 1 ->
    IAOR is already covered in booster classes... you might have missed these... you need to search for 'Physics Galaxy Booster Classes' in google or wait for re-stream schedule of these classes...
    7 years ago by Physics Galaxy
    Ans 2 ->
    Sir can you post link of PG' rigid body dynamics booster class video if possible It would be a great help
    7 years ago by Vivek Sindhwani

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  99.  
    100. Sir since v=rw hence v=3u/5L but will there be translational velocity(by linear momentum conservation as v'=u/3...???
    7 years ago by Shardul
    Ans 1 ->
    Here as rod is hinged, linear momentum is not conserved...
    7 years ago by Physics Galaxy

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  100.  
    101. sir plz tell wht all qus. cn be done from books like hcv nd irodov after each 2-3 lectures so we cn practice at home !
    7 years ago by Anant Jain
    Ans 1 ->
    H.C.Verma and D.C.Pandey books are enough brother
    6 years ago by Neelesh

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  101.  
    102. sir both F and friction are acting in clockwise direction so the torques should be added
    8 years ago by saikiran
    Ans 1 ->
    torque is taken about c
    7 years ago by Rithik Chopra

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  102.  
    103. how did you get 2v=dw; that certainly is incorrect!!
    8 years ago by Archana Sinha

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  103.  
    104. Sir in the above question why the momemt of inertia of the rod hinged at one end is taken as ML^2/12?why is it not taken as ML^2/3?
    8 years ago by Nikhil Bansal
    Ans 1 ->
    YES sir can you explain nikhil's question.
    7 years ago by swayamjeet

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  104.  
    105. sir while applying angular momentum conservation we must also MI of man in MI of disk as mn is also rotating along with it
    8 years ago by Dheeraj Agarwal

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  105.  
    106. sir why cant i do like this as MI of disk is 1/2mr^2 so of half dik it would be 1/4mr^2 (because MI is additive) now as some portion is cut so mi should be=1/4m(R1^2-R2^2) . as i can subtract moment of inertia of cutted disk
    8 years ago by Dheeraj Agarwal

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  106.  
    107. Won't we also have to take the translation kinetic energy of the sphere as it is in pure rolling motion?
    8 years ago by Daattavya Aggarwal

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  107.  
    108. are their no lectures on moment of force or torque??
    8 years ago by Vishal Ray
    Ans 1 ->
    These are there in the topic of Rigid Body Dynamics but the videos will be activated by coming weekend...
    7 years ago by Physics Galaxy
    Ans 2 ->
    Sir Pls upload the videos on torque...!!!!
    7 years ago by Akshunn Trivedi

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  108.  
    109. sir where is the description of torque
    8 years ago by Nimesh Dash
    Ans 1 ->
    Hmmm.
    6 years ago by aswin gawtham

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  109.  
    110. sir could you add video lecture of toppling
    8 years ago by harshit sharma
    Ans 1 ->
    Toppling is explained in detail in booster classes...
    7 years ago by Physics Galaxy

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  110.  
    111. sir angular velocity is (v1-v2 cos@)/R??? and moment of inertia of cylinder is mr2/2
    8 years ago by Mohammad Akram

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  111.  
    112. sir howcome height be L/2 and not L??
    8 years ago by Parash Goswami
    Ans 1 ->
    Distance are taken froM COM
    8 years ago by Gaurav Saharan

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  112.  
    113. How did u calculate the distance as ml/2/M+m??
    8 years ago by Parash Goswami

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  113.  
    114. how has the body fallen by a distance of 2R+R/4??pls explain it to me!!
    8 years ago by Parash Goswami
    Ans 1 ->
    actually the masss m has fallen by a distance of 2R+R/2 = 5R/4. you can see that he uses 5R/4 in the calculation part . he said 2R+R/4 in the video .
    8 years ago by Pranav Satheesh
    Ans 2 ->
    sorry i mean 2R+R/2=5R/2!!!
    8 years ago by Pranav Satheesh

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  114.  
    115. http://i.prntscr.com/a43246800dfa4f61a8671475ba7b2aa6.png Can someone check if the angular momentum is conserved properly or not?
    8 years ago by Satvik Sr
    Ans 1 ->
    acc to me we get w =3v/4a
    8 years ago by Satvik Sr
    Ans 2 ->
    during the action of impulsive forces all other finite forces can be neglected
    8 years ago by Mohammad Akram

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  115.  
    116. sir how can you conserve angular momentum about about AOR of cylinder??since weight of small m is acting is acting vertically downward.
    8 years ago by Mohammad Akram
    Ans 1 ->
    There is two reason behind this 1. Passing of line of impact of mg from the point about which we conserve momentum 2. Second one is cancelling out by producing same torque by normal I guess the first reason is most suitable for your doubt.
    8 years ago by Tushar Goyal
    Ans 2 ->
    hey 1)it's not passing through the point which we conserve momentum 2)if you are correct -normal reaction and weight are at same distance implies that they are equal..if they are equal then it should stay there itself i.e.it should not move but sir considered it is moving..thanks for repley
    8 years ago by Mohammad Akram
    Ans 3 ->
    In case of collisions compared to impulsive forces (short duration & high magnitude) all other finite forces can be neglected...
    8 years ago by Physics Galaxy
    Ans 4 ->
    torque produced by implusive forces ????
    8 years ago by Mohammad Akram

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  116.  
    117. If I rolls down a wheel then I notice that after some time it slows down and finally stop. What cause it to stop. I know that is friction but why it only acts backwardness forward.
    8 years ago by Tushar Goyal
    Ans 1 ->
    i think-- when we roll a wheel at first kinetic friction acts in backward direction and causes decelelration to the wheel and produces torque about center which incereases the angular acceleration and hence angular velocity..after some time v=rw therefore friction will be zero if you dont apply any force...now if there is no air resistance the wheel will be never come to rest (if the surface extends to infinity)..but in actual practise it exists threfore it comes to rest.
    8 years ago by Mohammad Akram
    Ans 2 ->
    No you said that kinetic friction is acting but it is performing rolling not slipping. Why friction only acts backward why not forward
    8 years ago by Tushar Goyal
    Ans 3 ->
    The correct reason behind this is that when the wheel comes in contact with floor it little bit compress and there is a normal force acting at some angle in leftward direction. And we know that the component of contact force in horizontal direction is called friction . That's why friction acts backward not forward. That is the correct reason behind this. It can be better understand by the help of diagram
    8 years ago by Tushar Goyal
    Ans 4 ->
    i think........ hey you said you will roll it implies that you have applied an external force..and the initial velocity both(angular,linear) is zero...rolling motion is of two types slipping,without slipping i.e. pure rolling...in the question you haven't said pure rolling..whenever there is relative sliding between two surfaces there will be kinetic friction..so there must be kinetic friction that decelerate forward motion incerease angular motion ..after some time the body starts pure rolling then kinetic friction vanishes also there is no force in horizontal line therefore static friction too becomes zero finally....it depends where you have applied the force on specific point.
    8 years ago by Mohammad Akram
    Ans 5 ->
    Pure rolling is nothing. It is just rolling. Either body rolls or slips. How they will perform both thing simultaneously. Think!
    8 years ago by Tushar Goyal
    Ans 6 ->
    It is the rolling friction which is due to the deformation of the surfaces in contact...
    8 years ago by Physics Galaxy
    Ans 7 ->
    If it is in pure rolling under ideal conditions (no deformation of surfaces) then the body will not stop... it will continue to roll forever as in absence of external forces in pure rolling friction is zero...
    8 years ago by Physics Galaxy
    Ans 8 ->
    thank you very much sir !!!
    8 years ago by Mohammad Akram
    Ans 9 ->
    due to the friction torque is produced...it also even reduces the velocity.....
    2 years ago by undefined
    Ans 10 ->
    I mean the linear velocity
    2 years ago by undefined

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  117.  
    118. Why net external torque about the hinge point is 0 and not any other point?
    8 years ago by Mostafijur Rahaman
    Ans 1 ->
    It is because if we consider any other point present on the rod than than calculate the torque then the torque of hinge force will come. Which implies that external torque is not zero. Therefore we consider hinge point as hinge force passes through it, therefore it's torque is zero
    8 years ago by Tushar Goyal

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  118.  
    119. What if the sphere is rotating about its own axis?
    8 years ago by Mostafijur Rahaman

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  119.  
    120. sir what about the torque due to the rotation of disk?
    8 years ago by gaurav sahu

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  120.  
    121. how to calculate distance on the plank between two impact of the ball if it is given angular velocity after 1st impact is zero?
    8 years ago by pratik yadav

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  121.  
    122. In previous example too the boy was at rest initially before he jumped and hence his angular momentum was taken as zero.But in this example he starts walking from rest but his initial angular momentum is taken as MR^2. Could you please explain.
    8 years ago by Ritwik Jha
    Ans 1 ->
    In the previous example initially the platform and the boy as a system were at rest...recall that the platform recoiled with an angular velocity w(omega)...in order to conserve the angular momentum after the jump of the boy....so the initial angular momentum was '0' in that case..But in this case the platform is already rotating with an initial angular velocity w1...so it will have initial angular momentum...Hope It Helps!!!
    7 years ago by shreya sood

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  122.  
    123. is vi the velocity of cm??
    8 years ago by Raj Verma

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  123.  
    124. Why didnt we use the relation a1=R.alpha ?? We use this on most of the other questions ?
    8 years ago by vishwanath v pai
    Ans 1 ->
    Because in this case the tension is providing the force for linear motion of this system ...due to which it will already have some linear acceleration which is a1 in this case..and due to the torque provided by this tension...the tangential acceleration will also get added up...and it's overall linear acceleration will be the sum of the two in this case...
    7 years ago by shreya sood
    Ans 2 ->
    so a1 can't be equal to R alpha..hope it helps!!!!
    7 years ago by shreya sood

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  124.  
    125. advance illustration 4 of rigid body dynamics; moment of inertia of solid cylinder is m(r square)/2 but in illustration it is given as 2/5m(r square) which is MOI of solid sphere
    8 years ago by gsairishithreddy

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  125.  
    126. why was the work done by tension not taken ? !! net wrk done = (mg-T)h ?? .plz help
    8 years ago by vishwanath v pai
    Ans 1 ->
    here this is done according to the conservation of mechanical energy..
    8 years ago by Khushboo Jain
    Ans 2 ->
    generally work done by tension is 0. I have said generally because in some case it is possible that tension might work.
    8 years ago by Tushar Goyal
    Ans 3 ->
    bcoz tension is an internal force of this system
    8 years ago by Karan Singh
    Ans 4 ->
    As due to tension the point of application of tension is not getting displaced work done by tension is zero.
    7 years ago by Viren Haldwania

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  126.  
    127. the answer comes out to be v=(5gh/14(sec0)^2-5)^1/2 not v=(5gh/(14(sec0)^2-1)^1/2
    8 years ago by Anshul Sharma
    Ans 1 ->
    0=theta
    8 years ago by Anshul Sharma
    Ans 2 ->
    It means 0 denotes theta since there is no theta sign on keyboard
    8 years ago by Anshul Sharma

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  127.  
    128. sir would it not be fh/2-Nx?
    8 years ago by AKASH SHARMA
    Ans 1 ->
    YEAH! HE IS WRONG!
    8 years ago by Pranav Satheesh

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  128.  
    129. sir in angular momentum impulse equation should Iomega not be added to LHS because at t=0 at has angular velocity omega?
    8 years ago by AKASH SHARMA
    Ans 1 ->
    angular momentum only gets increases after it starts silding due to the friction and as such initially there is no rolling....Initially we are dropping down the ball and due to the friction the ball strats rotating and as such there is an increase in the angular momentum.
    2 years ago by undefined

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  129.  
    130. If w final(omega is zero) then w initial = -alpha*t which gives t as negative..but this can happen...please explain..
    8 years ago by Ritwik Jha
    Ans 1 ->
    i mean w final(omega) is zero..
    8 years ago by Ritwik Jha
    Ans 2 ->
    here we have taken acceleration to be retardation which is a=(-ve)value used so time will be +ve
    8 years ago by Sangita Kumari

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  130.  
    131. why we havn't took the linear energy of com i.e 1/2 m v-square
    8 years ago by ronak gupta

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  131.  
    132. Sir...if the disc has a hole .. Then will moment of inertia will be affected or not...????
    8 years ago by Ritik Dhinwa
    Ans 1 ->
    Yes MOI will be affected
    8 years ago by Tushar Goyal
    Ans 2 ->
    i think........yes it is affected but formula format remains same
    8 years ago by Mohammad Akram

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  132.  
    133. We haven't studied torque earlier before......Here Sir is saying that we have already studied torque earlier..... No lecture on torque in all previous lectures.....
    8 years ago by Shantanu Roy
    Ans 1 ->
    Torque is the cross product of force at a particular point and the position vector w.r.t the axis of rotation
    8 years ago by Venkat S Raghavan
    Ans 2 ->
    learn by yourself friend!
    8 years ago by Aditya Gupta
    Ans 3 ->
    u have studied moment of mass naa. torque is nothing but moment of force. that's goodluck boy
    8 years ago by Sangita Kumari

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  133.  
    134. cant we use conservation of energy to get the angular speed. By the equation 1/2(m)u^2=1/2(I)w^2+1/2(m+M)v^2
    8 years ago by Akshat Gupta
    Ans 1 ->
    it is an inelastic collision so you cant conserve energy.
    8 years ago by harshit

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  134.  
    135. what is the moment of inertia of half disc about its c.o.m perpendicular to plane
    9 years ago by rohit pandey
    Ans 1 ->
    You can calculate it using parallel axis theorem... using Ic = I(about central axis) - M(4R/3pi)^2 where I(about central axis) = 0.5MR^2
    9 years ago by Physics Galaxy

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  135.  
    136. dear sir should we not write w=v/(R+r)?? As sphere is also rotating about the centre of big sphere?plz help me....
    9 years ago by manish reddy yedulla
    Ans 1 ->
    r is incomparable to R
    8 years ago by Saurabh kumar nayak

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  136.  
    137. Sir can you explain that how to evaluate problems involving torque in basic terms?
    9 years ago by Guru Raj Vaishnav

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  137.  
    138. Sir since impulse is acting on rod is linear momentum conserved???
    9 years ago by Guru Raj Vaishnav
    Ans 1 ->
    It is an internal force and internal forces do not affect the linear momentum of the system!!
    9 years ago by purnima nathani
    Ans 2 ->
    ya obviously since it is an an internal force so the total linear momentum is conserved
    8 years ago by Saurabh kumar nayak

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  138.  
    139. Consider a 3 dimentional rigid body at rest and acted upon by forces whose vector sum is zero. so, centre of mass will be at rest. there are many axes about which torque is not zero. which axes will be chosen by body for rotation? if the axis of rotation is through centre of mass, then there are several axes passing through centre of mass. which axis will the body choose to rotate? If torque about an axis passing through centre of mass is zero, there will be many other axes about which the torque is not zero. Will body rotate about any of these axes? which axis among them?
    9 years ago by Abhijit
    Ans 1 ->
    you will take the torque about axis passing through com only , but first you calculate the net torque. you now know the direction of angular accelaration( obviously in the direction of torque), so the axis will be parrallel to angular accelaration and passing through com ( if the body is initially in rest,w=0)
    8 years ago by Saurabh kumar nayak

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  139.  
    140. Suppose there is no pivot or pre-determined axis for a rigid body. There are many forces acting on the body. Then which axis will the body choose for rotation? I feel ( i am not dure) that in the most general situation, centre of mass of body with have some linear motion. The body will have some rotational motion whose axis will vary with time. Let's see some simplified situations. Suppose we are in 2 dimentional space. body is planar and all forces are on the same plane. vector sum of all forces will determine linear acceleration and hence linear motion of centre of mass. Then consider the frame of reference of centre of mass. From this frame, the motion is purely rotational. So we find torque about the center of mass. That gives us purely rotational motion of the body as seen from frame of centre of mass. if we see from inertial frame of reference, then body is rotating while its axis of rotation is translating with time. Now take a more general case of 3 dimensional rigid body acted upon by several non-collinear, non-coplanar forces. linear acceleration of centre of mass found by vector addition of all forces. then consider frame of reference of centre of mass. From this frame of reference, the motion will appear purely rotational. At a given time, the axis of rotation is passing through centre of mass and tangential to the motion of centre of mass. (I guess so. i am not sure if i am correct. Please advise.)
    9 years ago by Abhijit

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  140.  
    141. a typical proof or problem in rigid body dynamics starts with diagram showing a rigid body and a few forces acting on it along various lines. Then the analysis of its (linear and rotational) motion is done. The given situation of forces can be interpreted in two ways. (a) lines of application of forces are fixed rigidly on the paper. Imagine that body starts motion (linear cum rotational) with respect to paper. Still the forces do not change their line of application with respect to paper. (b) lines of application of forces are fixed with the body. when the body starts moving, the lines of application of forces move rigidly with the body. Which of the scenario (a) or (b) is assumed while deriving typical formulas of rigid body dynamics and in numerical problems?
    9 years ago by Abhijit

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  141.  
    142. In Atwood's machine why tension is different in two parts of string and what is friction has to do with it..??
    9 years ago by
    Ans 1 ->
    no assuming the string is massless draw a fbd of the string u will t1 not equal to t2 as frictional force comes into pic
    8 years ago by V Naga Malleswara Rao

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  142.  
    143. What is moment of inertia of a cylinder about its diagonal? Please answer
    9 years ago by Kushal Patankar
    Ans 1 ->
    how can a cylinder have diagonal???
    9 years ago by Bharath Ram
    Ans 2 ->
    I meant the line which is inclined at an angle of tan^{-1} (2r/h) with the vertical.where h is the height and r is radius of cylinder.
    9 years ago by Kushal Patankar

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  143.  
    144. Sir should'nt W1 (omega 1) be v/(R+r)?
    9 years ago by Rufa
    Ans 1 ->
    NO... as R is the radius of circle in which centre of sphere is moving...
    9 years ago by Physics Galaxy
    Ans 2 ->
    NO... as R is the radius of circle in which centre of sphere is moving...
    9 years ago by Physics Galaxy

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  144.  
    145. sir in this question i think the first equation should be equal to F-f=5ma as the force is being applied on plank of mass 4m but sphere is also present on the top of it which has a mass m.Sir please solve this query..
    9 years ago by Kanav
    Ans 1 ->
    sir wrote this equation by considering the plank as a system....by drawing the free body daigram of the plank and the cylinder u ll be able to get ur answer...
    8 years ago by Khushboo Jain
    Ans 2 ->
    sir it is a cylinder not a sphere!
    8 years ago by Pranav Satheesh
    Ans 3 ->
    Sir for the rotational equation shouldn't acceleration of cylinder be a2-a1. Thus the equation should be- fR=(MR^2)/2*(a2-a1)/r
    7 years ago by Akshunn Trivedi

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  145.  
    146. How can we assure that minimum normal will be when small mass is at the top. it is clear that it will be in the upper half. Potential energy of system is increasing and kinetic energy is decreasing as it is going up. my doubt is two things happening simultaneously velocity of ring is decreasing and centrifugal force is becoming more and more vertical
    9 years ago by AKASH GOYAL
    Ans 1 ->
    When the mass is not at the top or in upward motion on top... the centrifugal force will be in outward direction and passing through the bottom contact as it is instantaneous aor... only its upward component of this force will reduce the normal reaction which will be min when it reaches the top... analytically you can calculate and prove it also...
    9 years ago by Physics Galaxy

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  146.  
    147. Can we write the torque equation about the center of ring which is not the center of mass in this case.
    9 years ago by AKASH GOYAL
    Ans 1 ->
    yes. we can find torque about any axis, not necessarily passing through centre of mass, not necessarily passing through the body.
    9 years ago by Abhijit

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  147.  
    148. why couple of forces is independent from location of asis of rotation
    9 years ago by shivam sharma
    Ans 1 ->
    As in couple two equal and opposite forces act at some separation which are parallel to each other so in calculation of torque due to these forces... the distance from the axis of rotation will cancel out as both forces apply torque in opposite direction... so this torque only depends upon the separation between lines of action of forces...
    9 years ago by Physics Galaxy
    Ans 2 ->
    comment on Ans 1 -: The answer is ok is axis is perpendicular to the plane of couple. What if it is not? i feel - in general if we take a family of parallel axes, then the torque due to couple of forces about any of the axis within the family is same. Please correct me if i am wrong.
    9 years ago by Abhijit

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  148.  
    149. what is difference between couple of forces and torque
    9 years ago by shivam sharma
    Ans 1 ->
    Couple is torque due to two equal and opposite forces which are not acting along same line of action.... Also the couple is independent from the location of axis of rotation of body...
    9 years ago by Physics Galaxy

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  149.  
    150. A rectangular plate ABCD of mass m and dimensions a×b is supported in a vertical plane by two hinges at A and B. Find the instantaneous reaction of the hinge at A after B is removed. Sir i tried but could not solve. Please help
    9 years ago by Mohak Sharma
    Ans 1 ->
    When B is removed, the plate starts rotation about A. Just at the point of start you can write equation of motion for centre of mass of body in horizontal and vertical direction by taking two forces on hinge A Fx and Fy... To solve these equations you also need to write the equation of angular motion of plate about A which will be torque of mg = I(alpha) then by relating alpha and ax and ay you can get Fx and Fy...
    9 years ago by Physics Galaxy

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  150.  
    151. Sir i am unable to find solution of this problem A uniform rod is released from rest at an angle of 60 degree with the horizontal surface. Find the minimum static coefficient of friction between the rod and the surface so that the rod does not slip at the instant it is released.
    9 years ago by Mohak Sharma
    Ans 1 ->
    At the time of release take N and f as normal force and friction on rod at the point of contact. Now write the equations of motion of com of rod for its tangential acceleration and torque by mg = I (alpha) now you can use a(tan) = (l/2)(alpha) and find N and f. Now use f=(mu)N for sliding condition and get the value of mu...
    9 years ago by Physics Galaxy

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  151.  
    152. sir why did we use integration while calculating friction ,as friction is independent of area in contact morever we didnt use integration to calculate friction in translational motion
    9 years ago by vishal
    Ans 1 ->
    Here we are not integrating friction... we have to calculate the torque due to friction which is acting at different radii so to calculate the total torque of friction we need to integrate the torque due to friction at an elemental ring and we integrate as at different elements of disc torque due to friction is different..
    9 years ago by Physics Galaxy
    Ans 2 ->
    thanks a lot sir I have got your point
    9 years ago by vishal

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  152.  
    153. Q) A student throws a stick of length L up in the air. At the moment the stick leaves his hand , the speed of the sticks end is zero. The stick completes N turns just as its caught by the student at the initial release point . Show that the height h to which the centre of mass of the stick rose is h= (pi) NL/4
    9 years ago by Mohak Sharma
    Ans 1 ->
    Use omega.(2v)/g = N(2pi) and as initial velocity of end point is zero use omega (L/2) = v and solve for v and find height of com as h = v^2/2g... this gives the result.
    9 years ago by Physics Galaxy

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  153.  
    154. sir ihave done the same example in the way as integration of dm r^2 answer is not coming
    9 years ago by Aditi Gupta
    Ans 1 ->
    You cannot use integration of dmr^2 as that is valid by considering a point mass in the body as an element. Here element is a disc so you need to use the expression for MI of a disc using parallel axes theorem..
    9 years ago by Physics Galaxy
    Ans 2 ->
    sir why have you taken limits from -L/2 to L/2 ?
    6 years ago by Dhruv Malhotra

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  154.  
    155. sir, kindly tell me in solved example no 12, why while calculating Lf,why did we minus it and why not we added it as we did in the previous problems?
    9 years ago by Vineet Chatterjee
    Ans 1 ->
    Which solved example you are mentioning... If it is related to a video... pl post your question in the space given below the video...
    9 years ago by Physics Galaxy

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  155.  
    156. why is the two wheeler is able to balance itself while moving and not when its stationary?
    9 years ago by Vibhu Sharma
    Ans 1 ->
    This happens in moving state of a 2 wheeler because the angluar momentum of rotating wheels is in horizontal direction and no torque is there to change it so for the rider, its easy to balance... More details of this concept you will study under gyroscopic effect...
    9 years ago by Physics Galaxy

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  156.  
    157. find the moment of inertia of a cylinder of mass M raDIUS
    9 years ago by R Yashwanth
    Ans 1 ->
    Mr^2/2 about an axis perpendicular to its flat surface
    9 years ago by Lalitha

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  157.  
    158. Sir,how to do problems based on inelastic collision between a ball and a disc?
    9 years ago by Visisht

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  158.  
    159. sir, kindly tell me why while calculating Lf,why did we minus it and why not we added it as we did in the previous problems?
    9 years ago by Vineet Chatterjee
    Ans 1 ->
    mad dont know this much also analyse it u will get it
    9 years ago by mukhtar
    Ans 2 ->
    As boy is walkingAs boy is walking in opposite direction... his speed with respect to ground will be u - Rw2 and we use conservation of angular momentum in the frame of ground...
    9 years ago by Physics Galaxy

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  159.  
    160. in solved eg. 12 of rigid body mechanics, won't the L(final) be Iw2-m(u+Rw2)R??
    9 years ago by Balaji Aathithan
    Ans 1 ->
    NO. As boy is walking in opposite direction... his speed with respect to ground will be u - Rw2 and we use conservation of angular momentum in the frame of ground...
    9 years ago by Physics Galaxy

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  160.  
    161. A thin rod of length L and area of cross section S is pivoted at its lowest point P inside a stationary , homogeneous , non viscous liquid as shown in the figure. The rod is free to rotate in a vertical plane about a horizontal axis passing through P. The density of the material of the rod D(1) is smaller than the density of liquid D(2). The rod is displaced by a small angle ? from its equilibrium position and then released. IF YOU THINK IT WILL PERFORM S.H.M. Find its angular frequency. DETAILS: D(1) = 200 SI UNITS D(2) = 400 SI UNITS L = 20 cm g =10m/
    10 years ago by abdulmuttalib

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  161.  
    162. sir could you please explain about torque with some examples
    10 years ago by sudhanshu sharma
    Ans 1 ->
    torque is just a distance dependent vector quantity which dominates the entire rotational mechanics....G=r×F=rFsin(?)
    2 years ago by undefined

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  162.  
    163. what is instantanous axis of rotation
    10 years ago by santosh khadka
    Ans 1 ->
    it just acts as an rotational axis or it is even considered as the temporary rotational axis...
    2 years ago by undefined

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  163.  
    164. ?dmrcos? =0 for all bodies or only symmetric bodies
    10 years ago by rohith vaddavalli
    Ans 1 ->
    Here dmrcostheeta is the mass moment of the body w.r.to the centre of mass...and mass moment of any body w.r.to the centre of mass is always 0...
    7 years ago by shreya sood
    Ans 2 ->
    https://www.physicsgalaxy.com/lectures/1/12/200/How-to-Locate-Centre-of-Mass You can watch this video if you are having trouble with mass moments and why it gets zero w.r.t to COM.
    7 years ago by Akshunn Trivedi

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  164.  
    165. sir.... length should be taken... root2 L or ... 2L ...??? I m a bit confused...! !
    10 years ago by Ocean Balodiya
    Ans 1 ->
    For the MI of a square plate about its diagonal we use the length of its edge and its MI = (1/12) ML^2 so here L = sqrt(2)l
    9 years ago by Physics Galaxy

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  165.  
    166. sir can we complete the triangle to get a square and then find its I about COM and then parallel theorem? answer doesnt match then
    10 years ago by Agnivah Poddar
    Ans 1 ->
    with integration its more easy
    9 years ago by ameyayerpude
    Ans 2 ->
    NO. It cannot be done as the two triangles are not identical with respect to given Axis of rotation...
    9 years ago by Physics Galaxy

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